Hey all,
While trying to get a commit pushed for Yesod[1], Alexander Dunlap
pointed out one of his programs didn't work with the new code. After
some investigation, I was able to reproduce the bug with the following
code snippet:
{-# LANGUAGE TypeFamilies #-}
data family Foo a
data Bar = Bar
data instance Foo Bar
= Bar1 | Bar2 | Bar3 | Bar4 | Bar5 | Bar6 | Bar7 | Bar8 | Bar9
deriving Eq
This produces:
Couldn't match expected type `Main.R:FooBar'
against inferred type `Foo Bar'
NB: `Foo' is a type function
In the first argument of `Main.$con2tag_R:FooBar', namely `a'
In the expression: (Main.$con2tag_R:FooBar a)
In the expression:
case (Main.$con2tag_R:FooBar a) of {
a#
-> case (Main.$con2tag_R:FooBar b) of {
b# -> (a# GHC.Prim.==# b#) } }
The especially strange thing about this bug is that it only occurs
when there are more than 8 constructors; if I remove Bar9, everything
seems to work. Does anyone have experience with this occuring?
Michael
[1] http://docs.yesodweb.com/blog/please-break-yesod/